Camera modules for forming an image of a subject on a solid-state imaging device via a lens system are used widely for digital still cameras and mobile phone cameras. In recent years, camera modules have been required to have a larger number of pixels in combination with a lower profile. In general, as the number of pixels is increased, a lens system is required to have a higher resolution. Therefore, the thickness of a camera module tends to increase in the optical axis direction.
In this regard, when a pixel pitch of a solid-state imaging device is reduced, it is possible to reduce the imaging apparatus in size while keeping the same number of pixels. Based on this, an attempt has been made to enable the downsizing of a lens system and realize a camera module that combines a larger number of pixels with thinness. However, since the sensitivity and saturation power of a solid-state imaging device are in proportion to a pixel size, there is a limit to decreasing a pixel pitch.
Meanwhile, a compound-eye type camera module, which is a camera module having a plurality of units, each including a lens system and an imaging region on a solid-state imaging device, has been proposed to realize a reduction in thickness. For example, an exemplary compound-eye type camera module is proposed in Patent Document 1.
FIG. 13 is a view showing a configuration of a major portion of the compound-eye type camera module described in Patent Document 1. A lens array 132 includes three lenses 131a, 131b, and 131c. On the subject side of the respective lenses, a green spectral filter 133a, a red spectral filter 133b, and a blue spectral filter 133c are arranged.
A solid-state imaging device 134 includes therein three imaging regions for respectively forming images of a subject from the lenses. A green spectral filter 135a, a red spectral filter 135b, and a blue spectral filter 135c are provided also on the front surfaces of the respective imaging regions. With this configuration, it is possible to obtain a synthesized image by calculating signals obtained with respect to the respective units, each dealing with any one of green, red, and blue.
As compared with a camera module including only one pair of a lens system and a solid-state imaging device, a compound-eye type camera module as described above in which an equal-sized solid-state imaging device is divided into a plurality of imaging regions is capable of making an image smaller due to its individual divided imaging regions, so that a focal length of a lens system is reduced.
For example, in the case where a solid-state imaging device is divided into four equal portions lengthwise and crosswise, four lens systems are required. However, the focal length of the lens systems can be reduced to half, so that the thickness of a camera module can be reduced to half in the optical axis direction.
When each of the lens systems of the units is composed of a single lens rather than a plurality of combined lenses, it is possible to make a camera module thinner. However, when the lens system is composed of a single spherical lens, an aberration remains, which leads to insufficient resolution. For this reason, it is desirable to use at least an aspherical lens for the lens system.
Meanwhile, as a lens capable of achieving a still higher resolution than an aspherical lens, a diffractive lens is known that is provided with a concentric diffraction grating pattern on its aspherical lens surface. In addition to achieving a refraction effect of an aspherical lens, a diffractive lens can reduce various aberrations such as chromatic aberration significantly due to superimposed diffraction effects. When a diffractive lens with a diffraction grating pattern having a cross section in a blazed form or a fine-step-like form inscribed in a blaze is used, the diffractive lens is allowed to have a diffraction efficiency in a specific order of approximately 100% with respect to a single-wavelength light.
FIG. 14 is a view showing a conventional diffraction grating pattern. A diffraction grating pattern 142 is formed on a surface of a substrate 141 having a refractive index n(λ). Theoretically, a depth d of the diffraction grating pattern such that an m-th-order diffraction efficiency with respect to a light beam 143 that has a wavelength λ and is incident on the diffraction grating pattern 142 is 100% is given as the following formula.d=mλ/(n(λ)−1)  Formula (1)where the refractive index n(λ) is a function of the wavelength.
According to Formula (1), the value of d that gives an m-th-order diffraction efficiency of 100% varies as the wavelength λ varies. Although the following description is directed to the case of a 1st-order diffraction efficiency where m is 1, m is not limited to 1.
FIG. 15 is a diagram showing wavelength dependence of a 1st-order diffraction efficiency of a conventional diffraction grating. The figure shows a 1st-order diffraction efficiency with respect to a light beam that is incident on a diffraction grating pattern vertically. The diffraction grating pattern is formed on a cycloolefin-based resin of ZEONEX (produced by Zeon Corporation) and has a depth of 0.95 μm. The depth d of the diffraction grating pattern is designed with respect to a wavelength of 500 nm according to Formula (1). Therefore, the diffraction efficiency for 1st-order diffracted light is approximately 100% at a wavelength of 500 nm.
However, the 1st-order diffraction efficiency is about 75% at wavelengths of 400 nm and 700 nm due to its wavelength dependence. A decrease of 1st-order diffraction efficiency from 100% causes unnecessary diffracted light such as 0th-order, 2nd-order, and −1st order diffracted light.
In this manner, when a single diffractive lens is used over an entire visible light range (wavelengths of 400 to 700 nm), unnecessary diffracted light is likely to be generated. On the other hand, when the three lenses 131a, 131b, and 131c shown in FIG. 13 are used, a wavelength width of light to be handled by each of the lenses may be about 100 nm.
As shown in FIG. 15, at wavelengths of 450 to 550 nm, the 1st-order diffraction efficiency is about 95% or more, reaching its peak at a center wavelength of 500 nm, and unnecessary diffracted light is less likely to be generated. On this account, when a diffraction grating is used for each of the three lenses 131a, 131b, and 131c in FIG. 13, a depth d of the diffraction grating pattern may be adjusted as appropriate in accordance with the wavelength of green, red, or blue light according to Formula 1.
It is very effective to use a diffractive lens as a lens of a compound-eye type camera module in obtaining a high-resolution image. Hereinafter, a diffractive lens to be used mainly for imaging is referred to specifically as a diffractive imaging lens.
However, when a wide-angle image is captured, light from a subject is incident at a large angle with respect to an optical axis of a lens. According to the study by the present inventors, when a wide-angle image is captured by using the conventional diffractive imaging lens as described above, the image has a lower contrast.
FIG. 16 is a view showing a location of a light beam incident on a conventional diffraction grating. A substrate 161 is made of ZEONEX as described above, and a depth d of a diffraction grating pattern 162 is 0.95 μm, which is designed with respect to a wavelength of 500 nm according to Formula (1). An incident angle θ indicates an angle at which a light beam is incident on the diffraction grating pattern 162. FIGS. 17A to 17D show the wavelength dependence of a 1st-order diffraction efficiency in the diffraction grating in FIG. 16 with the incident angle θ being a parameter. FIGS. 17A, 17B, 17C, and 17D show wavelength dependence of a 1st-order diffraction efficiency in the cases where light is incident vertically, where θ is 10°, where θ is 20°, and where θ is 30°, respectively.
FIGS. 17A to 17D show that as the incident angle θ becomes larger, the wavelength at which the 1st-order diffraction efficiency is maximum is shifted to the long wavelength side. When considering light having a wavelength of 500 nm, when the depth d of the diffraction grating pattern is designed with respect to a wavelength of 500 nm according to Formula (1), a 1st-order diffraction efficiency of approximately 100% can be achieved if the light is incident vertically. However, when the incident angle θ varies to 10°, 20°, and 30°, the 1st-order diffraction efficiency decreases to 99.8%, 98.3%, and 91.5%, respectively. Namely, when it is attempted to obtain a wide-angle image, unnecessary diffracted light is generated, resulting in a decrease in resolution.
On the other hand, as can be seen from FIGS. 17A to 17D, at a wavelength of about 540 nm, the 1st-order diffraction efficiency is 98% or more even when the incident angle θ varies, and a decrease in 1st-order diffraction efficiency with an increase in incident angle θ is suppressed.
In view of the above, the depth d of the diffraction grating pattern is calculated and designed with respect to a wavelength λ that is set to be shorter than the wavelength to be used actually according to Formula (1). In other words, the depth of the diffraction grating pattern is made smaller than that to be calculated from the wavelength to be used actually. Consequently, with respect to a limited narrow wavelength range, a 1st-order diffraction efficiency of approximately 100% can be achieved irrespective of the incident angle as long as it is in a range of about 30°. The dependence of the diffraction efficiency on the incident angle as described above is seen not only when the diffraction grating pattern is formed on the flat plate as shown in FIG. 16 but also when it is formed on a spherical or aspherical lens.
As described above, in order to obtain a wide-angle image by using the conventional diffractive imaging lens, it is necessary to limit the wavelength width of light to be incident on the diffractive imaging lens to about 20 to 30 nm by allowing the light to pass through a filter having a narrow transmission wavelength band or the like. Accordingly, an amount of light received by a solid-state imaging device is decreased, and the S/N ratio is degraded. As a result, image quality is degraded particularly under dim lighting conditions.    Patent Document 1: JP 2001-78217 A